1. Field of the Invention
The invention generally relates to a method and apparatus for dividing the flow of a gas stream. More particularly, the invention relates to a method and apparatus for controlling the flow division of a corrosive conditioning gas stream.
2. Brief Description of Related Technology
In so-called flue gas conditioning applications, a conditioning gas, frequently diluted in hot air, is injected into a flue gas duct by means of a pipeline and a distribution manifold that supplies equal portions of the gas mixture to an array of parallel injection probes. Each probe itself is an extension of the distribution manifold, and is typically a capped pipe with a series of ports (e.g., holes) formed along the sides of the pipe, frequently directed at a tangent to the flue gas flow direction in the duct, to inject substantially equal portions of the gas through each hole. The injection manifold and array of probes are designed to use the pressure drop through the apparatus to distribute gas evenly through all of the parallel paths into the duct. The manifold is typically designed to make pipelines symmetrical when more than one flue gas duct is treated using a single source of conditioning agent.
Typical conditioning agents include sulfur trioxide and ammonia, which are used in effluent gas treatment applications, for example in fossil fuel-fired boilers such as may be found in electric power generating plants. These agents can be used in methods of treating flue gas to improve the efficiency of fly ash collection by electrostatic precipitators or bag houses, for example. Another application is the use of ammonia in systems designed to remove nitrogen oxides from flue gas, including selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) methods. Frequently, such as in these methods, it is important that the rate at which the conditioning agent is added to the flue gas can be controlled to respond to changes in the flue gas properties or quantity of flue gas.
In an electricity generating plant, for example, it often occurs that the plant is equipped with two or more nearly identical boilers arranged in parallel with nearly identical effluent gas treatment systems similarly arranged. In other plants, the boilers can have a common effluent gas stream divided and fed into two or more smaller effluent treatment systems of equal or different capacity arranged in parallel and in close proximity to one another. Thus, it is commonly found that there is the opportunity to supply a conditioning agent, such as sulfur trioxide, from a single source to more than one injection location. In such cases, it is necessary to divide (equally or unequally) the conditioning agent gas stream in a controlled manner so that a desired proportion of the conditioning agent is supplied to each injection location. In addition, it is necessary to actively control the apportionment (i.e., division and assignment) of the conditioning agent stream to respond to changes in demand for the conditioning agent, for example by changes in flue gas properties, by differing boiler operating rates, differing effluent gas flows though treatment systems, or during startup or shutdown of one or more boilers.
Previous attempts have been made to control the division of sulfur trioxide-containing gas streams using flow measuring sensors and flow control valves. However, such flow control valves and flow sensors, when exposed to hot sulfur trioxide-containing gas streams, have serious reliability problems. Many of the flow sensing technologies cannot be used at elevated operating temperatures. In an application for flue gas conditioning using gaseous sulfur trioxide, the sulfur trioxide is typically diluted in hot air and maintained at a temperature above the dew point of sulfuric acid (e.g., about 500° F.) to avoid condensation of sulfuric acid or oleum (a sulfur trioxide/sulfuric acid mixture). Likewise, the pipelines and flow control valves must be insulated and may be heated to avoid such condensation. Such control valves can fail by seizing, for example caused by corrosion and fouling of the control valve stem. Similarly, flow sensors can be damaged by sulfuric acid and/or oleum condensation and fouling. Condensation of sulfuric acid is one mechanism that is believed to contribute to this type of failure, though other factors, such as the extreme temperature variations that the valves and sensors experience, can also contribute to failure. Similarly, condensation of solid ammonium salts from an ammoniacontaining conditioning gas stream can foul valves and sensors used to control the flow of such a stream.